Brake shoes



United States Patent ABSTRACT OF THE DISCLOSURE A method of producing asuperior railroad brake shoe, in the form of a sintered body of powderedmetal including iron and graphite, by subjecting the sintered body torepeated pressure-no pressure cycles of the order of 3000 psi. at anelevated temperature and in a time period of less than 1.5 minutes.

This invention relates to an improved railroad car brake shoe and, moreparticularly, to such a brake shoe formed by a sintering process andexhibiting improved mechanical properties such as desirable frictionalproperties as well as higher strength than previously attainable. Thisapplication is a continuation of application Ser. No. 313,799, filedOct. 4, 1963, now abandoned.

In order to more clearly disclose the novel aspects of this invention, abrief discussion of the field of art to which it pertains is deemed tobe pertinent. For a great many years, the only material used as thefriction producing body to brake railroad cars was metallic in natureand was composed of cast ferrous alloys. These brake shoes have beenusually provided with a steel back around and upon which the castmaterial has been founded. This may be illustrated, for example, byUnited States Patent No. 1,157,183. The vast majority of railroad carsoperating in interchange service in the United States, Canada and Mexicoare provided with such brake shoes.

More recently, improvements in the technology of forming and newlyavailable materials have permitted the use of essentially organicallybonded brake shoes to be used for certain types of railroad cars and incertain types of service. These so-called composition brake shoes haveusually been composed of thermosetting resins, elastomers, asbestosfiber and certain other inorganic fillers and friction modifying agents.

Among many differences which exist between these two types of brakeshoes, and perhaps the most important from a functional viewpoint, isthe difference in their frictional properties. The cast metal brakeshoe, when evaluated under conditions specified by the Association ofAmerican Railroads (A.A.R.), is required to possess a mean coefficientof friction of at least 0.15 and a wear rate not to exceed 3.85 cubicinches of brake shoe material per 100,000 foot pounds of energy absorbedin braking effort. Under similarly specified conditions, the compositionbrake shoe has a mean coeflicient of friction of about 0.36 and a wearrate of about 0.5 cubic inch per 100,000 foot pounds. These propertiesare determined by testing brake shoes against a full size railroad carwheel mounted upon an appropriate dynamometer.

Because of these vastly different coeificients of friction, the brakerigging on a car equipped with composition brake shoes must be modifiedso that the load applied to these shoes during the braking action ismuch lower than that applied to the cast metal shoes. This is necessaryto avoid serious trouble during the braking of a train, parice ticularlyin cases where some cars in the train are equipped with compositionbrake shoes and the others with cast metallic brake shoes. Because ofthis, it is readily apparent that care must be exercised in replacingworn brake shoes to prevent placing cast metal shoes on a car whosebrake rigging has been modified for composition shoes, or equallydangerous, composition shoes on a car with standard rigging. In thefirst case, when the brakes are applied, inadequate braking action willtake place. In the second case, the braking action is excessive to thepoint where the wheels may lock resulting in producing fiat spots on thewheels, or, more drastically, in pulling the train apart with a possiblederailment.

Over the many years of their use, the cast metal brake shoes have beenexposed to practically every environmental condition it is possible toimagine and, except for normal wear during the braking function, hasproven to be virtually indestructible. During the much briefer historyof the composition brake shoe it has been found to be somewhat moreprone to damage. In particular, one of the more troublesome propertiesof the composition shoe is its lack of resistance to fire. It is acommon practice during winter in the northern states for railroad yardemployees to use torches to thaw frozen or ice-covered portions of cars.If composition brake shoes are exposed to flames for very long, theorganic material may be seriously damaged or destroyed; another cause ofexcessive wear and damage to composition shoes may originate in anexcessively rough wheel tread surface. The composition material isrelatively soft and such a tread condition will abrade it away at arelatively rapid rate, whereas the cast metal shoe is relatively immune.

For these and other reasons, it has become desirable to provide a brakeshoe having the advantages inherent in the metallic brake shoe withthose inherent in the composition brake shoe. Further, it is desirablethat such an improved brake shoe be interchangeable with existingcomposition brake shoes without further modification of the brakingsystems of the not inconsiderable number of cars already modified forcomposition brake shoes. In addition, it would also be desirable toprovide such a brake shoe with even higher frictional properties for newcar construction.

It is therefore, a principal object of this invention to provide arailroad brake shoe having essentially metallic characteristics andhigher friction properties than previously known cast metal brake shoes.A further object of this invention is the provision of such a brake shoehaving a mean coefficient of friction at least as high as existingcomposition brake shoes. A yet further object of this invention is theprovision of such a brake shoe composed essentially of sintered ironpowders and powdered graphite which may include friction augmenting ormodifying agents, which shoe, exhibits a higher mechanical strength thanthat previously producible. Other and different objects of thisinvention will become apparent from the following description, whereinit is to be understood that changes in the precise embodiment of theinvention can be made within the scope of what is claimed withoutdeparting from the spirit thereof.

Briefly stated and in accordance with one aspect of the invention, arailroad car brake shoe is produced by sintering and hot forming amixture of from about 60 to by weight iron powder, from about 10 to 25%by weight powdered graphite and up to 20% by weight of frictionmodifying and augmenting agents under conditions to provide the brakeshoe with a mean coefiicient of friction of at least 0.30 and a wearrate of less than 3.85 cubic inches per 100,000,000 foot pounds ofenergy absorption, and having a transverse rupture strength of at least3,000 pounds per square inch.

The following detailed description of the several compositions andmethods employed in achieving such an improved and novel railroad carbrake shoe are set forth for the purpose of more completely disclosingthe various aspects of the invention and are merely exemplary and not tobe construed as limiting the scope of the invention except as may be setforth in the several claims.

Essentially the inventive subject matter involves the provision of arailroad car brake shoe composed of a wrought and appropriately shapedsteel back provided with lugs and guide members where desired to adaptit and the attached body or bodies of friction material to be mountedupon the brake head of a railroad car. In order for the improved brakeshoe to accomplish the purposes and objects of the invention, thefriction material attached to the steel back must have certainmechanical and frictional properties which are dependent upon itscomposition and the manner in which it is formed. As will be morespecifically described subsequently, it has been found that such a brakeshoe may be constructed by forming a plurality of steel backed segmentsof friction material and attaching them by bolting, riveting, or anyappropriate means to a steel brake shoe which may then be attached to abrake head or, more desirably, by forming an integral mass of frictionmaterial in contact with and securely afiixed to a steel brake shoe backas a single, monolithic structure to be attached to a brake head.

More specifically, appropriate proportions of iron powder, graphite andperhaps other ulverulent materials, as will be subsequently disclosed,are thoroughly mixed in the dry state, placed in an appropriatelyconfigured die and pressed at room temperature under loads of from about5 tons per square inch to as high as about 14 tons per square inch toform a cold, compacted selfsupporting body. The compacted powder body isthen removed from the forming die and an appropriate steel back isassembled therewith and the two piece assembly is placed within aprotective atmosphere furnace and heated to at least 1900 F. for about 1hour. The assembled body and steel back are then removed from thefurnace and placed immediately in an appropriately formed forming dieand subjected to a cyclically applied pressure by means of a punch-likemember which substantially fills and closes the upper open end portionof the die cavity. This loading is accomplished by applying a pressureof 3,000 pounds per square inch for 7.5 seconds and releasing thepressure for 1 second and then repeating the cycle for a total of 8cycles. The composite body is then removed from the die and cooled.Preferably the cooling is done by inserting the body into a container ofsolid carbon dioxide (Dry Ice) whereby the rate of cooling isaccelerated and a protective, nonoxidizing atmosphere is provided. Thefinished body is composed of a layer of compacted and sintered frictionmaterial securely fastened or welded to the steel back and suitable foruse as a segmented railroad brake shoe, as previously described or, ifthe sintered friction material has been secured directly to a wroughtsteel railroad brake shoe back member, as a monolithic railroad brakeshoe. The apparatus for accomplishing this is well known and a similarprocess is described and illustrated in United States Patent No.2,892,707, hereinafter referred to as the Biggs patent. It should benoted, however, that the above-recited process differs in severalimportant details from that disclosed in the Biggs patent. The patentdiscloses the use of a specifically different friction composition and,additionally, a much lower pressure and a substantially greater numberof cycles employed during the pressing operation. As will be shown ingreater detail, the pressure of 3,000 pounds per square inch and thesignificantly lower number of cycles utilized in the practice of thisinvention results in an unexpectedly higher transverse rupture strengthand the composition used has frictional properties which enable theresulting material to be used for rail-cad braking purposes.

It should be noted, particularly in view of the fact that the sinteredpowder metal friction material art is quite well developed, that whensintering such iron base materials under pressure, i.e. utilizing theapplication of pressure to the powder compact during the sinteringoperation, 1800 F. has proven to be quite adequate as a sinteringtemperature. Furthermore, in such materials the inclusion of as much as20 percent by weight of lead powder in the compact has been shown tohave virtually no effect upon the transverse rupture strength whencompared to the same material without the lead.

Example 1 A mixture of dry powders having the following composition wasprepared:

After thoroughly mixing the powders at room temperature, they werepressed at room temperature in a die at a pressure of 5 tons per squareinch to form a selfsustaining compact having a configuration andapproximate dimensions of a full-size standard railroad brake shoe. Thiscompact was then removed from the die and a wrought steel brake shoeback placed upon it. The two so-assembled bodies were then placed in afurnace and heated for 1 hour at 1900 F. A protective atmosphere wasgenerated in the furnace by the presence of charcoal the-rein. At theend of the 1 hour period, the compact and back were removed, still inthe assembled relationship, and immediately placed in the hot formingdie having the dimensions and configuration of a fullsize railroad brakeshoe and subjected to a cyclically applied compacting pressure of 3,000pounds per square inch. The pressure cycle employed applied the 3,000pounds per square inch load for 7.5 seconds and then released the loadfor 1 second. This cycle was repeated for a total of 8 such cycles. Theso-formed brake shoe was then removed from the die and placed in acontainer with solid carbon dioxide blocks (Dry Ice) and permitted tocool. Subsequently, this brake shoe was mounted on a brake head in adynamometer in which a standard railroad car wheel was installed and itsfrictional properties and wear rate was determined under standard A.A.R.specified conditions. In this test, an equivalent wheel load of 7,324pounds, a brake shoe load of 2,500 pounds and a 36 inch diameter wheelwere used. Under these conditions, the brake shoe produced adynamorneter stop distance of 800 feet or less from 60 miles per hour.After five preliminary stops to wear in or sea the brake shoe, theresults of which were discarded, a series of 15 stops were made from 60miles per hour. It was found from these tests that this brake shoe had amean coefficient of friction of 0.49 and a wear rate of 1.11 cubicinches per 100,000,000 foot pounds of energy absorbed. Test specimensmeasuring 1" x 2" x 0.2" were cut from the sintered friction materialportion of the brake shoe and subjected to a standard three pointtransverse rupture test. The material exhibited a transverse rupturestrength of 4,060 pounds per square inch.

Example 2 A mixture of powders having a composition identical to that ofExample 1 was mixed and processed in the same manner and by the sameequipment as that of Example 1 except it and its associated steel backwere heated for 1 hour at 1800 F. instead of 1900 F., and it wassubjected to a hot forming operation consisting of 16 cycles of 1,500pounds per square inch applied for 4 seconds followed by a release ofpressure for 1 second. In all other respects the brake shoe wasidentically treated to that produced in Example 1. It was subjected tothe same testing procedure on the dynarnometer and was found to have amean coefficient of friction of 0.44 and a wear rate of 1.17 cubicinches per 100,000,000 foot pounds of absorbed energy. When similar testspecimens were cut from the friction material of this brake shoe andsubjected to the same transverse rupture strength test it was found tohave a strength of only 2,540 pounds per square inch.

Example 3 ample 1 except that the diesused produced flat plate-like orblock-like bodies of sintered friction material about 7" long by 4% wideby 1" thick having a 7" by 4 by ,5 thick steel back secured to one facethereof. Additionally, a higher cold pressing pressure of 14 tons persquare inch was employed. A plurality of these blocks were secured to astandard steel brake shoe back and trimmed and ground to present a wearface substantially identical in size and shape to that of the brake shoeof Example 1. When tested upon the dynamometer as set forth in Example1, this segmental shoe was found to have a mean coefficient of frictionof 0.40 and a wear rate of 0.43 cubic inch per 100,000,000 foot pounds.This friction material had a transverse rupture strength of 4,800 poundsper square inch.

Example 4 A mixture of powders having a composition identical to that ofExample 1 was mixed and processed in the same manner and by the sameequipment as that of Example 3 except that the heating temperature, hotforming pressure and pressure cycle set forth in Example 2 was used.Transverse rupture test specimens were cut from the friction materialand upon test were found to have a transverse rupture strength of 2,540pounds per square inch.

In order to illustrate the relationship and effect of the severalvariables upon the strength of this type of friction material, a numberof fiat blocks having steel backs attached thereto were made in a diemeasuring 3%" x 4%" from a mixture consisting of 72% by weight ironpowder, 18% by weight graphite powder and by weight lead powder. Theywere all cold compacted under a pressure of 14 tons per square inch,heated for 1 hour and hot formed as indicated, cooled with Dry Ice aspreviously set forth and transverse rupture strength test specimens cutfrom each were tested with the indicated results.

From the foregoing, it is apparent that there is a relationship betweenthe various variables involved which indicates that the highertemperature of 1900 F., the higher forming pressure of 3,000 pounds persquare inch and the 7.5 second on to 1 second olf cycle for 8 cyclescombine to produce optimum transverse rupture strength. This is alsoborne out by the comparative rupture strength of the previous examples.

As might be expected, variations in composition of these materials havean effect upon their friction and wear properties and to some extentupon their strength, which is illustrated in the following tables.

TABLE 2 Composition, Percent by Weight Fe Graphite Pb SiC MOS: LIulliteSn TABLE 3 T.R.S., p.s.i. M.C.F. Wear, On. In.

As is evident from inspection, Table 2 illustrates the compositionalvariables. Segmental brake shoes were made from each of these materialsin the same manner as set forth in Example 3 and tested as set forth inExamples 1 and 3. The results of these tests are shown in Table 3.

From the foregoing disclosure it will be apparent that useful railroadbrake shoes may be manufactured from compositions which are essentiallyiron powder and powdered graphite which have much higher strengths andfrictional and wear properties which are compatible to or better in somerespects than the properties of existing composition shoes and whichpossess advantages which the composition shoes cannot match.

While for the purposes of this disclosure, certain specific exampleshave been set forth, it is to be understood that this invention is notto be limited thereby in any way except as set forth in the appendedclaims. For example, one obvious variation in the processing would be toform the cold pressed compact with the steel backing member in the dieor to omit the steel back member completely during the hot formingoperation.

Hence, while preferred embodiments have been illustrated and described,it is to be understood that these are capable of variation andmodification.

We claim:

1. A method of producing a railroad brake shoe having a body presentingan effective wear portion of compacted .and sintered particlesconsisting essentially of from about 60 to about 75 percent by weightiron powder, from about 10 to about 25 percent by weight graphite powderand up to about 20 percent by Weight of powdered friction and augmentingagents, said portion of the shoe having a mean coefiicient of frictionof at least 0.30 and a wear rate of less than 3.85 cubic inches per100,- 000,000 foot pounds of absorbent energy as determined under A.A.R.dynamometer test procedures, comprising: heating the body of compactedand sintered particles to at least 1700 F. and subjecting the heatedbody to a pressure of about 3000 pounds per square inch applied througha plurality of pressure-no pressure cycles for a time not exceeding 1.5minutes while the body is confined within a die.

2. A method according to claim 1 wherein the pressure-no pressure cyclesinvolve the application of about 3000 pounds per square inch for aperiod of up to 10 seconds followed by a period during which no pressureis applied for about 1 second.

3. A method according to claim 2 in which pressure is applied for about7.5 seconds, in which the total number of cycles is 8, and in which thetemperature of the body subjected to pressure is in the range of about1800 F. to 1900 F.

4. A method according to claim 1 in which the powders consistessentially of from about 65 to 72 percent by weight of iron, about 10to 20 percent by weightof graphite, up to about 20 percent by weight oflead, up to about percent by weight of tin, up to about percent byweight of Alundum, up to about 7.5 percent by weight of silicon carbide,up to about percent by weight of mullite, and up to about 5 percent byWeight of molybdenum disulphide.

5. A method according to claim 3 in which the powders consist of aboutto 71 percent iron, 2 to 3 percent tin, 1 to 3 percent lead, 4 to 6percent Alundum, and from 19 to 21 percent graphite.

References Cited UNITED STATES PATENTS Schaefer 206 X Biggs 75208 Cox29182.8 Stedman et a1. 75206 Ankeny et al. 29-182.5 Luther et a1. 75208X Smiley 75-208 X Weinman et a1. 75-201 X Quartullo 75226 X Storchheirnet al. 29-420.5 X Schlomer et a1. 75201 X 15 CARL D. QUARFORTH, PrimaryExaminer.

R. L. GRUDZIEOKL'Assistr/mt Examiner.

1. A METHOD OF PRODUCING A RAILROAD BRAKE SHOE HAVING A BODY PRESENTINGAN EFFECTIVE WEAR PORTION OF COMPACTED AND SINTERED PARTICLES CONSISTINGESSENTIALLY OF FROM ABOUT 60 TO ABOUT 75 PERCENT BY WEIGHT IRON POWDER,FROM ABOUT 10 TO ABOUT 25 PERCENT BY WEIGHT GRAPHITE POWDER AND UP TOABOUT 20 PERCENT BY WEIGHT OF POWDERED FRICTION AND AUGMENTING AGENTS,SAID PORTION OF THE SHOE HAVING A MEANS COEFFICIENT OF FRICTION OF ATLEAST 0.30 AND A WEAR RATE OF LESS THAN 3.85 CUBIC INCHES PER100,000,000 FOOT POUNDS OF ABSORBENT ENERGY AS DETERMINED UNDER A.A.R.DYNAMOMETER TEST PROCEDURES, COMPRISING: HEATING THE BODY OF COMPACTEDAND SINTERED PARTICLES TO AT LEAST 1700* F. AND SUBJECTING THE HEATEDBODY TO A PRESSURE OF ABOUT 3000 POUNDS PER SQUARE INCH APPLIED THROUGHA PLURALITY OF PRESSURE-NO PRESSURE CYCLES FOR A TIME NOT EXCEEDING 1.5MINUTES WHILE THE BODY IS CONFINED WITHIN A DIE.